1. Field of the Invention
The present invention relates to a method of manufacturing an active matrix substrate using a method of transferring a thin film device. The present invention also relates to an active matrix substrate manufactured by the manufacturing method, and a liquid crystal display device comprising this active matrix substrate as one of a pair of substrates.
2. Description of Related Art
For example, a liquid crystal display using thin film transistors (TFT) is manufactured through the step of forming thin film transistors on a substrate by CVD or the like. Since the step of forming thin film transistors on the substrate is accompanied with high temperature processing, it is necessary to use material for the substrate which has excellent heat resistance, i.e., material having a high softening point and melting point. At present, silica glass is used as a substrate which can resist a temperature of about 10000C, and heat resistant glass is used as a substrate which can resist a temperature of about 500xc2x0 C.
Namely, the substrate on which thin film elements are mounted must satisfy conditions for producing the thin film transistors. Therefore, the substrate used is determined so as to satisfy conditions for manufacturing a device to be mounted thereon.
However, in consideration of only the steps after the substrate comprising the thin film transistors such as TFT or the like mounted thereon is completed, in some cases, the above-described substrate is not always satisfactory.
For example, in the above-described manufacturing process accompanied with high temperature processing, a quartz substrate, a heat-resistant substrate, or the like is used. However, these substrates are very expensive, and thus cause an increase in product cost.
Also the glass substrate has the properties that it is heavy and easily broken. A liquid crystal display used for portable electronic apparatus such as a palm top computer, a portable telephone, etc. is preferably light weight, can resist a little deformation, and is hardly broken by dropping. However, in fact, the glass substrate is generally heavy, less resistant to deformation and is possibly broken by dropping.
In other words, there are gaps between the limitations caused by manufacturing conditions and preferable characteristics required for products, and it is very difficult to satisfy the conditions and characteristics.
The present invention has been achieved in consideration of these problems, and an object of the invention is to provide a novel technique which permits independent free selection of a substrate used in producing thin film devices, and a substrate (a substrate having preferable properties for application of a product) used in, for example, actual use of a product, and a completely new method of effectively manufacturing an active matrix substrate having excellent properties and a liquid crystal display device by using the technique.
In order to achieve the object, the present invention may include the following.
(1) The present invention provides a method of manufacturing an active matrix substrate comprising a pixel portion including thin film transistors connected to scanning lines and signal lines arranged in a matrix, and pixel electrodes respectively connected to terminals of the thin film transistors, the method may include:
forming a separation layer on the substrate;
forming the thin film transistors over the separation layer;
forming an insulation film on the thin film transistors and over the separation layer;
selectively removing at least a portion of the insulation film in a region where each of the pixel electrodes is to be formed;
forming each of the pixel electrodes on the insulation film and the separation layer in a region where at least a portion of the insulation film has been removed;
adhering the thin film transistors to a transfer material with an adhesive layer
producing exfoliation in the separation layer and/or at an interface of the separation layer and the substrate to separate the substrate from the separation layer; and
removing any portion of the separation layer remaining on the pixel electrodes and under the insulation film to form an active matrix substrate using the transfer material as a new substrate.
In the method of manufacturing an active matrix substrate of the present invention, the thin film transistors and the pixel electrodes formed on the substrate are transferred to the desired transfer material by the device transfer technique developed by the applicant of the present invention. In this case, the device transferred onto the transfer material is reverse to a normal device. In the transferred device, consequently, the pixel electrode is covered with the insulator layer such as an interlayer insulation film or the like before transfer. If the insulation film has a large thickness, a large voltage loss occurs in this portion, and thus a sufficient voltage cannot be applied to a liquid crystal.
Therefore, in the manufacturing method of the present invention, in forming the thin film transistors and pixel electrodes on the original substrate before transfer, at least a portion of the insulator layer such as the interlayer insulation film or the like is removed before the pixel electrodes are formed. In this case, the entire insulator layer is preferably removed. However, when the insulation film remaining unremoved is thin, at least a portion of the insulator layer may be removed because no problem occurs in application of a voltage to the liquid crystal.
In any case, by separating the original substrate after a device is transferred onto the transfer material, the pixel electrode partially appears at least in the vicinity of the surface of the device. Therefore, a sufficient voltage can be applied to the liquid crystal layer from this portion.
The insulation film remaining on the pixel electrodes can also be separately removed in another step (for example, in a step after transfer of the device).
(2) The present invention provides a method of manufacturing an active matrix substrate comprising a pixel portion including thin film transistors connected to scanning lines and signal lines arranged in a matrix, and pixel electrodes respectively connected to terminals of the thin film transistors, and the method may include:
forming a separation layer on a substrate;
forming an intermediate layer on the separation layer;
forming the thin film transistors on the intermediate layer;
forming an insulation film on the thin film transistors and the intermediate layer;
selectively removing a portion of the insulation film in a region where each of the pixel electrodes is to be formed;
forming each of the pixel electrodes on the insulation film and the separation layer in the region where at least a portion of the insulation film is removed;
adhering the thin film transistors to a transfer material with an adhesive layer;
producing exfoliation in the separation layer and/or at an interface of the separation layer and the substrate to separate the substrate from the separation layer; and
removing any portion of the separation layer remaining on the intermediate layer and the pixel electrodes to form an active matrix substrate using the transfer material as a new substrate.
This invention is different from invention (1) in that the intermediate layer is provided. The intermediate layer can comprise a single layer film of an insulator, such as an SiO2 film or the like, or a multilayered film comprising a laminate of an insulator and a metal. The intermediate layer functions to facilitate separation from the separation layer, protect the transistors from contamination during removal of the separation layer, ensure insulation properties of the transistors, and suppress irradiation of the transistors with laser light.
In forming the thin film transistors and the pixel electrodes on the original substrate before transfer, at least a portion of the insulator layer such as an interlayer insulation film or the like, which causes a problem in the later steps, is removed before the pixel electrodes are formed. In this case, the whole insulation film and intermediate layer below it are preferably removed at the same time from the viewpoint of prevention of a loss of the voltage applied to the liquid crystal. However, where the insulator layer remaining unremoved is thin, a sufficient voltage can be applied to the liquid crystal from the pixel electrodes. Therefore, at least a portion of the insulation film may be removed.
In the present invention, by separating the original substrate after a device is transferred to the transfer material, the pixel electrode partially appears at least in the vicinity of the surface of the device. Therefore, a voltage can sufficiently be applied to the liquid crystal layer from this portion.
The insulation film remaining on the pixel electrodes can separately be removed in another step (for example, the step after transfer of the device).
(3) In invention (2), at least a portion of the insulation film may be selectively removed in the step of forming contact holes for electrically connecting the pixel electrodes to the thin film transistors. Since the same manufacturing step is used for both purposes, an increase in the number of the manufacturing steps can be prevented.
(4) In invention (3), the contact holes may be used for connecting the pixel electrodes directly to an impurity layer which constitutes the thin film transistors.
Namely, in a structure in which the pixel electrodes are connected directly to terminals(source layer or drain layer) of the thin film transistors, the insulator layer such as an interlayer insulation film or the like may be removed in formation of the contact holes for connection.
(5) In invention (3), the contact holes may be used for connecting the pixel electrodes to respective electrodes connected to an impurity layer which constitutes the thin film transistors.
Namely, in a structure in which the pixel electrodes are connected to terminals(the source layer or drain layer) of the thin film transistors through electrodes made of a metal or the like (when the pixel electrodes are in a layer above the electrodes of the transistors), the insulator layer such as an interlayer insulation film or the like may be removed in formation of the contact holes for connection.
(6) In any one of inventions (1) to (5), at least one of a color filter and a light shielding film may be after the step of forming the pixel electrodes.
In the structure of normal thin film transistors, if the color filter or the light shielding film is formed on the pixel electrodes, application of a voltage to the liquid crystal layer from the pixel electrodes is interfered with, and thus such a structure cannot be used.
However, in the present invention, a device is reversed by transfer, and thus the region where a voltage is applied to the liquid crystal layer from the pixel electrode is formed on the side (i.e., the thin film transistor side) opposite to a conventional device. Therefore, even if the color filter or the light shielding film has been previously formed on the original substrate before transfer, no trouble occurs. In this case, only common electrodes may be formed on the opposite substrate, and the color filter or the light shielding film, which is conventionally formed on the opposite substrate, need not be strictly aligned with the pixel electrodes, thereby facilitating assembly of a liquid crystal display device.
(7) In any one of inventions (1) to (6), in selectively removing at least a portion of the insulation film, at least a portion of the insulation film may be selectively removed in a region where an external connection terminal is to be provided.
In an active matrix substrate, where the external connection terminal (for example, a terminal for connecting a liquid crystal driving IC) is required, this terminal also must be at a position near at least the surface of the device.
Therefore, in the region where the external connection terminal is provided, the insulator film such as an interlayer insulation film or the like is removed. In this case, the under insulation film (intermediate layer) must be removed in the same step or a different step.
(8) In invention (7), in the region where at least a portion of the insulation film is selectively removed for providing the external connection terminal, a conductive layer formed from the same material as the pixel electrodes or an electrode connected to an impurity layer which constitutes the thin film transistors may be formed. In this invention, the conductive layer may be used for forming the external connection terminal.
(9) The present invention also may provide a method of manufacturing an active matrix substrate having a pixel portion including thin film transistors connected to scanning lines and signal lines arranged in a matrix, and pixel electrodes connected to terminals of the thin film transistors, and the method may include:
forming a separation layer on a transmissive substrate;
forming the thin film transistors over the separation layer or on a predetermined intermediate layer formed on the separation layer;
forming an insulation film on the thin film transistors;
forming the pixel electrodes comprising a transparent conductive material on the insulation film;
forming a light shielding layer that is overlapped with the thin film transistors and is not overlapped with at least a portion of the pixel electrodes;
adhering the thin film transistors and the light shielding layer on a transmissive transfer material with a transmissive adhesive layer;
irradiating the separation layer with light through the transmissive substrate to produce exfoliation in the separation layer and/or at an interface of the separation layer and the transmissive substrate to separate the transmissive substrate from the separation layer;
forming a photoresist on a surface from which the transmissive substrate is separated, or on the surface of a layer which appears after removing any remaining portion of the separation layer;
irradiating light to expose only a predetermined portion of the photoresist using the light shielding layer as a mask, followed by development to form a desired photoresist mask;
selectively removing at least a portion of the intermediate layer and the insulation film or at least a portion of the insulation film using the photoresist mask; and
removing the photoresist mask to form an active matrix substrate using the transfer material as a new substrate.
Although, in inventions (1) to (8), at least a portion of the insulator layer below the pixel electrodes may be removed before transfer, in this invention, at least a portion of the insulator layer below the pixel electrodes may be removed in a self alignment manner using the light shielding film after transfer.
Namely, the light shielding layer may be formed on the original substrate before transfer, and may be used as an exposure mask after transfer to form a desired resist pattern by utilizing the fact that the light shielding layer is formed around the pixel electrodes. Then, at least a portion of the insulator layer below the pixel electrodes may be removed by using the resist pattern as an etching mask.
(10) This invention provides a method of manufacturing an active matrix substrate having a pixel portion including thin film transistors connected to scanning lines and signal lines arranged in a matrix, and pixel electrodes respectively connected to terminals of the thin film transistors, and the method may include:
forming a separation layer on a substrate;
forming the pixel electrodes over the separation layer or on a predetermined intermediate layer formed on the separation layer;
forming an insulation film on the pixel electrodes, forming the thin film transistors on the insulation film, and respectively connecting the thin film transistors to the pixel electrodes;
adhering the thin film transistors to a transfer material with an adhesive layer;
producing exfoliation in the separation layer and/or at an interface of the separation layer and the substrate to separate the substrate from the separation layer; and
removing any portion of the separation layer remaining on the intermediate layer to form an active matrix substrate using the transfer material as a new substrate.
In this invention, when the thin film transistors are formed on the original substrate before transfer, the pixel electrodes are previously formed. The original substrate before transfer is separated after transfer to automatically expose the surfaces of the pixel electrodes or position the pixel electrodes at least at the surface of the device.
(11) In invention (10), a conductive material layer may be formed on the separation layer or on the intermediate layer at a position where an external connection terminal is to be formed.
When the thin film transistors are formed on the original substrate before transfer, the conductive material layer for forming the external connection terminal is previously formed as well as the pixel electrode. The original substrate before transfer is separated after transfer to automatically expose the surface of the conductive material layer at the same time as the pixel electrodes, or position the conductive material layer near the surface, leaving the intermediate layer. In the latter case, the intermediate layer is removed in the same step or a different step to expose the surface of the conductive material layer. The conductive material layer with the exposed surface serves as the external connection terminal.
(12) This invention provides an active matrix substrate manufactured by the method of manufacturing an active matrix substrate of any one of inventions (1) to (11). Since limitations due to the manufacturing conditions are eliminated so that the substrate can freely be selected, a novel active matrix substrate, which has not yet been realized, can be realized.
(13) This invention provides a liquid crystal display device comprising an active matrix substrate manufactured by the method of manufacturing an active matrix substrate of any one of inventions (1) to (11). For example, it is possible to realize an active matrix type liquid crystal display device comprising a plastic substrate and having flexibility.